# How to fix ImportError: C extension: No module named 'pandas._libs.tslibs.conversion' not built

You are importing pandas from a source checkout whose C extensions were never built. Either build them or just install a binary wheel with pip. Most people hitting this should do the pip reinstall: the error only appears when a source directory shadows a working install.

## The exact error

```text
ImportError: C extension: No module named 'pandas._libs.tslibs.conversion' not built
```

## Fix it

1. Check whether you are inside the pandas source dir: `pwd` and look for setup.py. If yes, cd out.
   Expected: you know whether the source tree is shadowing the install

2. Simplest fix: `python -m pip install --force-reinstall pandas`.
   Expected: `python -c "import pandas; print(pandas.__version__)"` works from any directory

3. If you actually develop pandas: build the extensions in place, then import again.
   Expected: the C extensions build and import succeeds

## When this applies

`import pandas` raises the 'C extension ... not built' ImportError naming a pandas._libs module

## When this does NOT apply

plain `No module named 'pandas'` or a missing optional dependency like openpyxl

## Versions

pandas built from source / dev installs, any version

## Why it happens

pandas ships compiled Cython extensions. A source checkout (or a copied pandas folder) without built extensions cannot import.

## Edge cases

Do not run your scripts from inside the pandas repo directory; the local pandas/ folder shadows site-packages.